diff --git a/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs
new file mode 100644
index 000000000000..573b3ed03801
--- /dev/null
+++ b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs	
@@ -0,0 +1,69 @@
+using System;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Testing.Controllers
+{
+    [Route("api/[controller]")]
+    [ApiController]
+    public class ClientSuppliedIpUsedInSecurityCheck : ControllerBase
+    {
+        [HttpGet("bad1")]
+        public IActionResult Bad1()
+        {
+            var ip = GetClientIp();
+            if (!ip.StartsWith("192.168."))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok("Bad");
+        }
+
+        [HttpGet("bad2")]
+        public IActionResult Bad2()
+        {
+            var ip = GetClientIp();
+            if (!ip.Equals("127.0.0.1"))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok("Bad");
+        }
+
+        [HttpGet("good1")]
+        public IActionResult Good1()
+        {
+            string ip = Request.Headers["X-Forwarded-For"];
+            // Good: if this application runs behind a reverse proxy it may append the real,
+            // remote IP to the end of any client-supplied X-Forwarded-For header.
+            ip = ip.Split(",")[ip.Split(",").Length - 1];
+
+            if (!ip.StartsWith("192.168."))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok();
+        }
+
+        private string GetClientIp()
+        {
+            var xfHeader = Request.Headers["X-Forwarded-For"];
+            if (!string.IsNullOrEmpty(xfHeader))
+            {
+                return xfHeader.ToString().Split(",")[0];
+            }
+
+            xfHeader = Request.HttpContext
+                .Connection.RemoteIpAddress?.ToString();
+
+            if (string.IsNullOrEmpty(xfHeader))
+            {
+                throw new Exception("ip not found");
+            }
+
+            return xfHeader;
+        }
+    }
+}
diff --git a/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qhelp b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qhelp
new file mode 100644
index 000000000000..dbb0168c253f
--- /dev/null
+++ b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qhelp	
@@ -0,0 +1,35 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>An original client IP address is retrieved from an http header (<code>X-Forwarded-For</code> or <code>X-Real-IP</code> or <code>Proxy-Client-IP</code> 
+etc.), which is used to ensure security. Attackers can forge the value of these identifiers to
+bypass a ban-list, for example.</p>
+
+</overview>
+<recommendation>
+
+<p>Do not trust the values of HTTP headers allegedly identifying the originating IP. If you are aware your application will run behind some reverse proxies then the last entry of a <code>X-Forwarded-For</code> header value may be more trustworthy than the rest of it because some reverse proxies append the IP address they observed to the end of any remote-supplied header.</p>
+
+</recommendation>
+<example>
+
+<p>The following examples show the bad case and the good case respectively.
+In <code>Bad1</code> method and <code>Bad2</code> method, the client ip the <code>X-Forwarded-For</code> is split into comma-separated values, but the less-trustworthy first one is used. Both of these examples could be deceived by providing a forged HTTP header. The method
+<code>Good1</code> similarly splits an <code>X-Forwarded-For</code> value, but uses the last, more-trustworthy entry.</p>
+
+<sample src="ClientSuppliedIpUsedInSecurityCheck.cs" />
+
+</example>
+<references>
+
+<li>Dennis Schneider: <a href="https://www.dennis-schneider.com/blog/prevent-ip-address-spoofing-with-x-forwarded-for-header-and-aws-elb-in-clojure-ring/">
+Prevent IP address spoofing with X-Forwarded-For header when using AWS ELB and Clojure Ring</a>
+</li>
+
+<li>Security Rule Zero: <a href="https://www.f5.com/company/blog/security-rule-zero-a-warning-about-x-forwarded-for">A Warning about X-Forwarded-For</a>
+</li>
+
+</references>
+</qhelp>
diff --git a/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.ql b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.ql
new file mode 100644
index 000000000000..aeb2aa579ee3
--- /dev/null
+++ b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.ql	
@@ -0,0 +1,41 @@
+/**
+ * @name IP address spoofing
+ * @description A remote endpoint identifier is read from an HTTP header. Attackers can modify the value
+ *              of the identifier to forge the client ip.
+ * @kind path-problem
+ * @problem.severity error
+ * @precision high
+ * @id csharp/ip-address-spoofing
+ * @tags security
+ *       external/cwe/cwe-348
+ */
+
+import csharp
+import ClientSuppliedIpUsedInSecurityCheckLib
+import DataFlow::PathGraph
+
+/**
+ * Taint-tracking configuration tracing flow from obtaining a client ip from an HTTP header to a sensitive use.
+ */
+class ClientSuppliedIpUsedInSecurityCheckConfig extends TaintTracking::Configuration {
+  ClientSuppliedIpUsedInSecurityCheckConfig() { this = "ClientSuppliedIpUsedInSecurityCheckConfig" }
+
+  override predicate isSource(DataFlow::Node source) {
+    source instanceof ClientSuppliedIpUsedInSecurityCheckSource
+  }
+
+  override predicate isSink(DataFlow::Node sink) {
+    sink instanceof ClientSuppliedIpUsedInSecurityCheckSink
+  }
+
+  override predicate isSanitizer(DataFlow::Node node) {
+    node instanceof ClientSuppliedIpUsedInSecurityCheckSanitizer
+  }
+}
+
+from
+  DataFlow::PathNode source, DataFlow::PathNode sink,
+  ClientSuppliedIpUsedInSecurityCheckConfig config
+where config.hasFlowPath(source, sink)
+select sink.getNode(), source, sink, "IP address spoofing might include code from $@.",
+  source.getNode(), "this user input"
diff --git a/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheckLib.qll b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheckLib.qll
new file mode 100644
index 000000000000..f7a7b7710c78
--- /dev/null
+++ b/csharp/ql/src/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheckLib.qll	
@@ -0,0 +1,170 @@
+private import semmle.code.csharp.security.dataflow.flowsources.Remote
+private import semmle.code.csharp.frameworks.microsoft.AspNetCore
+private import semmle.code.csharp.frameworks.System
+private import semmle.code.csharp.security.dataflow.SqlInjectionQuery
+
+/** A data flow source for ip address forgery vulnerabilities. */
+abstract class ClientSuppliedIpUsedInSecurityCheckSource extends DataFlow::Node { }
+
+/** A data flow sink for ip address forgery vulnerabilities. */
+abstract class ClientSuppliedIpUsedInSecurityCheckSink extends DataFlow::ExprNode { }
+
+/** A data flow sanitizer for ip address forgery vulnerabilities. */
+abstract class ClientSuppliedIpUsedInSecurityCheckSanitizer extends DataFlow::ExprNode { }
+
+/**
+ * A data flow source of the client ip obtained according to the remote endpoint identifier specified
+ * (`X-Forwarded-For`, `X-Real-IP`, `Proxy-Client-IP`, etc.) in the header.
+ *
+ * For example: `Request.Headers["X-Forwarded-For"]` or `request.ServerVariables["HTTP_X_FORWARDED_FOR"]`.
+ */
+private class AspNetQueryStringSource extends ClientSuppliedIpUsedInSecurityCheckSource,
+  DataFlow::ExprNode {
+  AspNetQueryStringSource() {
+    exists(RemoteFlowSource rfs, RefType rt, IndexerAccess ia, Call c |
+      rfs.asExpr() = ia.getQualifier() and
+      ia.getTarget() = rt.getAnIndexer() and
+      c.getTarget() = rt.getAnIndexer().getGetter() and
+      c.getArgument(0).(StringLiteral).getValue().toLowerCase() = clientIpParameterName() and
+      this.getExpr() = c
+    )
+    or
+    //  `Request.Headers.TryGetValue("X-Forwarded-For", out var xHeader)`
+    exists(RemoteFlowSource rfs, MethodCall mc |
+      rfs.asExpr() = mc.getQualifier() and
+      mc.getArgument(0).(StringLiteral).getValue().toLowerCase() = clientIpParameterName() and
+      this.getExpr() = mc
+    )
+  }
+}
+
+/**
+ * A data flow source of the client ip obtained according to the remote endpoint identifier specified
+ * (`X-Forwarded-For`, `X-Real-IP`, `Proxy-Client-IP`, etc.) in the header.
+ *
+ * For example: `[FromHeader(Name = "X-Forwarded-For")]`.
+ */
+private class AspNetCoreActionMethodParameterSource extends ClientSuppliedIpUsedInSecurityCheckSource,
+  DataFlow::ParameterNode {
+  AspNetCoreActionMethodParameterSource() {
+    exists(RemoteFlowSource rfs, Parameter p |
+      rfs.asParameter() = p and
+      p.getAnAttribute().getType().hasQualifiedName("Microsoft.AspNetCore.Mvc.FromHeaderAttribute") and
+      p.getAnAttribute().getNamedArgument("Name").(StringLiteral).getValue().toLowerCase() =
+        clientIpParameterName() and
+      this.getParameter() = p
+    )
+  }
+}
+
+/**
+ * A data flow sink for remote client ip comparison.
+ *
+ * For example: `if (!ip.StartsWith("192.168."))` determine whether the client ip starts
+ * with `192.168.`, and the program can be deceived by forging the ip address.
+ */
+private class CompareSink extends ClientSuppliedIpUsedInSecurityCheckSink {
+  CompareSink() {
+    exists(MethodCall mc |
+      mc.getTarget().hasQualifiedName("System.String", ["Equals", "Contains"]) and
+      mc.getTarget().getDeclaringType() instanceof StringType and
+      (
+        // ip.Equals("127.0.0.1")
+        this.getExpr() = mc.getQualifier() and
+        mc.getArgument(0).getValue() instanceof PrivateHostName and
+        not mc.getArgument(0).getValue() = "0:0:0:0:0:0:0:1"
+        or
+        // "127.0.0.1".Equals(ip)
+        this.getExpr() = mc.getArgument(0) and
+        mc.getQualifier().(StringLiteral).getValue() instanceof PrivateHostName and
+        not mc.getQualifier().(StringLiteral).getValue() = "0:0:0:0:0:0:0:1"
+      )
+    )
+    or
+    exists(ComparisonOperation co |
+      (co instanceof EQExpr or co instanceof NEExpr) and
+      this.getExpr() = co and
+      if co.getRightOperand().hasValue()
+      then (
+        // ip == "127.0.0.1"
+        co.getRightOperand().getType() instanceof StringType and
+        co.getRightOperand().(StringLiteral).getValue() instanceof PrivateHostName and
+        not co.getRightOperand().(StringLiteral).getValue() = "0:0:0:0:0:0:0:1"
+      ) else (
+        // "127.0.0.1" == ip
+        co.getLeftOperand().getType() instanceof StringType and
+        co.getLeftOperand().(StringLiteral).getValue() instanceof PrivateHostName and
+        not co.getLeftOperand().(StringLiteral).getValue() = "0:0:0:0:0:0:0:1"
+      )
+    )
+    or
+    exists(MethodCall mc |
+      mc.getTarget().hasQualifiedName("System.String", ["StartsWith", "Contains"]) and
+      mc.getTarget().getDeclaringType() instanceof StringType and
+      (
+        // ip.StartsWith("192.168.")
+        this.getExpr() = mc.getQualifier() and
+        mc.getArgument(0).(StringLiteral).getValue().regexpMatch(getIpAddressRegex())
+        or
+        // "192.168.".StartsWith(ip)
+        this.getExpr() = mc.getArgument(0) and
+        mc.getQualifier().(StringLiteral).getValue().regexpMatch(getIpAddressRegex())
+      )
+    )
+  }
+}
+
+/** A data flow sink for sql operation. */
+private class SqlOperationSink extends ClientSuppliedIpUsedInSecurityCheckSink {
+  SqlOperationSink() { this instanceof SqlInjectionExprSink }
+}
+
+/**
+ * Splitting a header value by `,` and taking an entry other than the first is sanitizing, because
+ * later entries may originate from more-trustworthy intermediate proxies, not the original client.
+ */
+private class SplitMethodSanitizer extends ClientSuppliedIpUsedInSecurityCheckSanitizer {
+  SplitMethodSanitizer() {
+    // Split(',')[0]
+    exists(ArrayAccess aa, MethodCall mc, SystemStringClass s |
+      this.getExpr() = aa.getQualifier() and
+      mc = aa.getQualifier() and
+      mc.getTarget() = s.getSplitMethod() and
+      not aa.getAnIndex().(IntLiteral).getValue() = "0"
+    )
+    or
+    // Split(',').FirstOrDefault()
+    exists(MethodCall mc, SystemStringClass s |
+      this.getExpr() = mc and
+      mc.getTarget() instanceof ExtensionMethod and
+      mc.getArgument(0).(MethodCall).getTarget() = s.getSplitMethod() and
+      not mc.getTarget()
+          .hasQualifiedName("System.Linq.Enumerable",
+            ["FirstOrDefault<System.String>", "First<System.String>"])
+    )
+  }
+}
+
+/**
+ * A string matching private host names of IPv4 and IPv6, which only matches the host portion therefore checking for port is not necessary.
+ * Several examples are localhost, reserved IPv4 IP addresses including 127.0.0.1, 10.x.x.x, 172.16.x,x, 192.168.x,x, and reserved IPv6 addresses including [0:0:0:0:0:0:0:1] and [::1]
+ */
+private class PrivateHostName extends string {
+  bindingset[this]
+  PrivateHostName() {
+    this.regexpMatch("(?i)localhost(?:[:/?#].*)?|127\\.0\\.0\\.1(?:[:/?#].*)?|10(?:\\.[0-9]+){3}(?:[:/?#].*)?|172\\.16(?:\\.[0-9]+){2}(?:[:/?#].*)?|192.168(?:\\.[0-9]+){2}(?:[:/?#].*)?|\\[?0:0:0:0:0:0:0:1\\]?(?:[:/?#].*)?|\\[?::1\\]?(?:[:/?#].*)?")
+  }
+}
+
+string clientIpParameterName() {
+  result in [
+      "x-forwarded-for", "x-real-ip", "proxy-client-ip", "wl-proxy-client-ip",
+      "http_x_forwarded_for", "http_x_forwarded", "http_x_cluster_client_ip", "http_client_ip",
+      "http_forwarded_for", "http_forwarded", "http_via", "remote_addr"
+    ]
+}
+
+string getIpAddressRegex() {
+  result =
+    "^((10\\.((1\\d{2})?|(2[0-4]\\d)?|(25[0-5])?|([1-9]\\d|[0-9])?)(\\.)?)|(192\\.168\\.)|172\\.(1[6789]|2[0-9]|3[01])\\.)((1\\d{2})?|(2[0-4]\\d)?|(25[0-5])?|([1-9]\\d|[0-9])?)(\\.)?((1\\d{2})?|(2[0-4]\\d)?|(25[0-5])?|([1-9]\\d|[0-9])?)$"
+}
diff --git a/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs
new file mode 100644
index 000000000000..573b3ed03801
--- /dev/null
+++ b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.cs	
@@ -0,0 +1,69 @@
+using System;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Testing.Controllers
+{
+    [Route("api/[controller]")]
+    [ApiController]
+    public class ClientSuppliedIpUsedInSecurityCheck : ControllerBase
+    {
+        [HttpGet("bad1")]
+        public IActionResult Bad1()
+        {
+            var ip = GetClientIp();
+            if (!ip.StartsWith("192.168."))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok("Bad");
+        }
+
+        [HttpGet("bad2")]
+        public IActionResult Bad2()
+        {
+            var ip = GetClientIp();
+            if (!ip.Equals("127.0.0.1"))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok("Bad");
+        }
+
+        [HttpGet("good1")]
+        public IActionResult Good1()
+        {
+            string ip = Request.Headers["X-Forwarded-For"];
+            // Good: if this application runs behind a reverse proxy it may append the real,
+            // remote IP to the end of any client-supplied X-Forwarded-For header.
+            ip = ip.Split(",")[ip.Split(",").Length - 1];
+
+            if (!ip.StartsWith("192.168."))
+            {
+                throw new Exception("illegal ip");
+            }
+
+            return Ok();
+        }
+
+        private string GetClientIp()
+        {
+            var xfHeader = Request.Headers["X-Forwarded-For"];
+            if (!string.IsNullOrEmpty(xfHeader))
+            {
+                return xfHeader.ToString().Split(",")[0];
+            }
+
+            xfHeader = Request.HttpContext
+                .Connection.RemoteIpAddress?.ToString();
+
+            if (string.IsNullOrEmpty(xfHeader))
+            {
+                throw new Exception("ip not found");
+            }
+
+            return xfHeader;
+        }
+    }
+}
diff --git a/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.expected b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.expected
new file mode 100644
index 000000000000..ab915389fdc2
--- /dev/null
+++ b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.expected	
@@ -0,0 +1,23 @@
+edges
+| ClientSuppliedIpUsedInSecurityCheck.cs:13:22:13:34 | call to method GetClientIp : Object | ClientSuppliedIpUsedInSecurityCheck.cs:14:18:14:19 | access to local variable ip |
+| ClientSuppliedIpUsedInSecurityCheck.cs:25:22:25:34 | call to method GetClientIp : Object | ClientSuppliedIpUsedInSecurityCheck.cs:26:18:26:19 | access to local variable ip |
+| ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer : StringValues | ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:31 | access to local variable xfHeader : StringValues |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:31 | access to local variable xfHeader : StringValues | ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:42 | call to method ToString : String |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:42 | call to method ToString : String | ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:53 | call to method Split [element] : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:53 | call to method Split [element] : Object | ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:56 | access to array element : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:56 | access to array element : Object | ClientSuppliedIpUsedInSecurityCheck.cs:13:22:13:34 | call to method GetClientIp : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:56 | access to array element : Object | ClientSuppliedIpUsedInSecurityCheck.cs:25:22:25:34 | call to method GetClientIp : Object |
+nodes
+| ClientSuppliedIpUsedInSecurityCheck.cs:13:22:13:34 | call to method GetClientIp : Object | semmle.label | call to method GetClientIp : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:14:18:14:19 | access to local variable ip | semmle.label | access to local variable ip |
+| ClientSuppliedIpUsedInSecurityCheck.cs:25:22:25:34 | call to method GetClientIp : Object | semmle.label | call to method GetClientIp : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:26:18:26:19 | access to local variable ip | semmle.label | access to local variable ip |
+| ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer : StringValues | semmle.label | access to indexer : StringValues |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:31 | access to local variable xfHeader : StringValues | semmle.label | access to local variable xfHeader : StringValues |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:42 | call to method ToString : String | semmle.label | call to method ToString : String |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:53 | call to method Split [element] : Object | semmle.label | call to method Split [element] : Object |
+| ClientSuppliedIpUsedInSecurityCheck.cs:55:24:55:56 | access to array element : Object | semmle.label | access to array element : Object |
+subpaths
+#select
+| ClientSuppliedIpUsedInSecurityCheck.cs:14:18:14:19 | access to local variable ip | ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer : StringValues | ClientSuppliedIpUsedInSecurityCheck.cs:14:18:14:19 | access to local variable ip | IP address spoofing might include code from $@. | ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer | this user input |
+| ClientSuppliedIpUsedInSecurityCheck.cs:26:18:26:19 | access to local variable ip | ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer : StringValues | ClientSuppliedIpUsedInSecurityCheck.cs:26:18:26:19 | access to local variable ip | IP address spoofing might include code from $@. | ClientSuppliedIpUsedInSecurityCheck.cs:52:28:52:61 | access to indexer | this user input |
diff --git a/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qlref b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qlref
new file mode 100644
index 000000000000..67afef9fabcd
--- /dev/null
+++ b/csharp/ql/test/experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.qlref	
@@ -0,0 +1 @@
+experimental/Security Features/CWE-348/ClientSuppliedIpUsedInSecurityCheck.ql
diff --git a/csharp/ql/test/experimental/Security Features/CWE-348/options b/csharp/ql/test/experimental/Security Features/CWE-348/options
new file mode 100644
index 000000000000..ef338c0b1c12
--- /dev/null
+++ b/csharp/ql/test/experimental/Security Features/CWE-348/options	
@@ -0,0 +1,2 @@
+semmle-extractor-options: /nostdlib /noconfig
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/_frameworks/Microsoft.AspNetCore.App/Microsoft.AspNetCore.App.csproj